1. Field of the Invention
This invention relates to an air/fuel ratio control system for an internal combustion engine the air/fuel ratio of an internal combustion which may hereinafter also be called an "engine" as needed.
2. Description of the Related Art
An exhaust gas purifying system is conventionally known wherein a three-way catalyst for purifying exhaust gas of an internal combustion engine is disposed in an exhaust system of the internal combustion engine to purify exhaust gas of the engine.
It is already known that the exhaust gas purifying efficiency of such an exhaust gas purifying system can be improved by fluctuating the air/fuel ratio around the theoretical air/fuel ratio.
To this end, an oxygen concentration sensor of the .lambda. type (which denotes an oxygen concentration sensor which presents a sudden change in output value thereof around a predetermined air/fuel ratio (theoretical air/fuel ratio, and such sensor will be hereinafter referred to as O.sub.2 sensor) is conventionally provided in an exhaust manifold, i.e., on an forward side of a catalytic converter. Interested with the fact that the output of such O.sub.2 sensor presents a change from an on-state to an off-state, that is, a change from a high voltage level to a low voltage level or vice versa across the predetermined air/fuel ratio (theoretical air/fuel ratio), the output of the O.sub.2 sensor is fed back to control the air/fuel ratio so that the air/fuel ratio may remain around the theoretical air/fuel ratio. Such control is called O.sub.2 feedback control.
In such O.sub.2 feedback control, an output of the O.sub.2 sensor is compared with an on/off threshold voltage (reference value), and if, for example, the O.sub.2 sensor output is higher than the threshold voltage, the air/fuel ratio is controlled toward the lean side, but on the contrary, if the O.sub.2 sensor output is lower than the threshold voltage, the air/fuel ratio is controlled toward the rich side.
It has recently been proposed to provide an additional O.sub.2 sensor on the rearward side of the catalytic converter provided in the engine exhaust system (This O.sub.2 sensor will hereinafter be called "rearward O.sub.2 sensor" while an O.sub.2 sensor provided on the forward side of the catalytic converter like the above-described O.sub.2 sensor will be called an "forward O.sub.2 sensor") and to information for the control of the air/fuel ratio (so-called dual O.sub.2 sensor system or double O.sub.2 sensor system). Even in this case, a standard value which should be compared with an output from the rearward O.sub.2 sensor will not be changed once it has been set.
It has also been proposed to arrange an O.sub.2 sensor, which has a slow detection response speed, on an forward side of a catalytic converter disposed in an engine exhaust system and to use an output from the O.sub.2 sensor as information for the correction of control of the air/fuel ratio.
Such conventional means however involve the following problems when the output of the O.sub.2 sensor indicates a rich air/fuel ratio as a result of control by the O.sub.2 sensor and the timing of acceleration in a small intake-air-quantity operation state (low-speed and low-load operation state, low-load operation state, idling state, or the like) before acceleration [see FIG. 52(a), point al]. Since the catalytic converter is in an oxygen-deficient state before such acceleration, acceleration as shown in FIG. 52(c) in such a state leads to the problem that the emission of HC and CO increases immediately after the acceleration [see the characteristic curve shown by a solid line in FIG. 52 (b). In addition, the catalytic converter is brought into an oxygen-excessive lean state because of the control by the O.sub.2 sensor after the acceleration [see FIG. 52(a), point a2]. This results in a reduction to the efficiency of purification of NOx, so that more NOx is emitted as shown by the dashed characteristic curve in FIG. 52(b).